Many domestic, industrial, agricultural and aqua-cultural processes result in effluents having a very high biological oxygen demand (BOD) which must be substantially reduced before the treated effluent can be finally discharged. Such effluents are typically associated with the preparation or consumption of food stuffs, for instance fluids from food preparation and sewage.
For quite some time it has been known that such effluents are nutrient rich and can be biologically de-graded by treatment with appropriate bacterial cultures. Such bacteria feed on the nutrients and destroy many of the waste substances.
It is well-known that the effectiveness of aerobic bacteria is dependent on the amount of dissolved oxygen in the effluent and that such bacteria prefer to grow on a fixed surface. In order to promote the activity of aerobic bacteria it is well-known to oxygenate the effluent and to provide a submerged matrix having a large surface area for colonisation by the bacteria. For instance, Smith and Loveless Inc of Lenexa, Kans., United States of America have marketed a domestic sewage treatment plant in which a shrouded matrix of corrugated plastic sheets defining cross-flow passages is suspended in a portion of a cesspit adjacent a discharge pipe for the treated effluent. An electric motor drives a submerged impeller to draw the sewage from the cesspit into the shroud above the matrix whereby the sewage then flows downwards through the matrix back into the cesspit. The motor also drives a fan to supply air to the agitated surface of the sewage above the matrix thereby oxygenating the liquid. Bacteria grows on the surfaces of the matrix until it eventually sloughs off and falls into the bottom of the cesspit where it joins other settled solids and then undergoes anaerobic bacterial degradation. This prior proposal will certainly reduce the BOD of the fluid leaving the system through the discharge pipe but its overall efficiency is impaired by the low rate of oxygenation and the mixing of the liquid leaving the bottom of the matrix with the contents of the cesspit. Another system being used is that developed by the Polybac Corporation of Allentown, Pa., United States of America and sold as the CTX Bioreactor. This bioreactor comprises a tank having a single matrix constructed of modules stacked side-by-side over a series of air pipes which are spaced about 50 cm apart and have large holes in their upper surfaces spaced about 5 cm apart. The air leaving these holes passes upwards through the cross-flow passages of the matrix causing the liquid to be uplifted over each pipe and then to flow downwards again through the matrix in positions between the pipes. In this manner the air supply, in addition to oxygenating the liquid, serves to pump the liquid upwards and downwards through the matrix with a vertical and a horizontal mixing action. In those regions of the matrices where the liquid is being pumped upwards by the air bubbles, the oxygenation promotes aerobic bacterial growth on the walls of the matrices and, when such bacterial growth is sufficiently thick, promotes its sloughing off the walls. However, the rate of oxygen absorption is low due to the large air bubbles, and the liquid passing downwards through the matrix essentially holds less oxygen. As a result the rate of bacterial growth in the downflow portions of the matrix is lower than in the upflow portions and this bacterial growth is not physically disturbed by the passage of air bubbles. As a result the slower bacterial growth in the downflow portions of the matrix tends to accumulate thereby progressively blinding the cross-flow passages in this portion of the matrix. This apparatus can promote significant BOD reduction by the time the liquid leaves the tank but the horizontal mixing of the liquid in between the liquid inlet and liquid outlet of the tank means that a proportion of the incoming fluid will reach the outlet without being adequately treated.
U.S. Pat. No 4,680,111 teaches a sewage treatment equipment with activated sludge process beds comprising a plurality of treatment tanks which are separated by bulkheads and dashboards such that the bottom of each treatment tank is connected by a passage behind its dashboard to a horizontal passage through its bulkhead into the top of an adjoining treatment tank. An aeration pipe extends across part of the floor of each treatment tank and is formed with slits through which air is blown out into the sewage water in the form of air bubbles. A series of activated sludge processing beds are located by support rods extending between the bulkhead and dashboard of each treatment tank and generally comprise a cylindrical core of hard synthetic resin surrounded by a porous member formed of corrosion resistant yarns intertwisted with spongy mesh-like or fibrous synthetic resin. These sludge processing beds are provided for the growth of both aerobic and anaerobic bacteria, and also of giant micro-organisms. The outer periphery of each sludge processing bed is a site for aerobic bacterial growth whilst anaerobic bacterial growth occurs inside due to the construction of the sludge processing beds restricting access to the air bubble. Indeed the construction of the sludge processing beds is varied from treatment tank to treatment tank so that the proportion of anaerobic bacteria increased progressively from 20% to 60% of the total bacterial growth. Various giant micro-organisms are introduced into the equipment to eat the accumulating bacterial growth on the sludge processing beds. This form of sewage treatment equipment is incapable of processing effluent by predominant aerobic bacterial action and is, to the contrary, intended to operate by a combined aerobic/anaerobic process of which the anaerobic component increases progressively through the treatment. The construction of the sludge processing beds are such that the bacterial growths will attach themselves so firmly to the twisted fibres that blinding can only be prevented by the introduction of organisms which will graze the accumulating bacterial growths. The slits in the aeration pipes will inherently form large air bubbles, and the size and positioning of each-aeration pipe in its chamber is such that the flow of air inevitably favours the central portion of each group of sludge processing beds.
It is an object of the present invention to provide a process for the treatment of effluent predominantly by aerobic bacteria which is of greater effectiveness, and also to provide apparatus for performing that process.